This invention relates to nuclear magnetic resonance (NMR) imaging methods and systems and, more particularly, to a method and apparatus for generating a maximum intensity projection image of a tortuous and non-planar vessel.
The present invention can be used with imaging techniques (e.g. NMR, positron emission tomography or PET, computerized tomography or CT, etc.) that generate a three-dimensional data point array which is then used to generate an image for viewing on a two-dimensional screen. To simplify the explanation, the invention is described in the context of an NMR system.
Any nucleus which possesses a magnetic moment attempts to align itself with the direction of the magnetic field in which it is located. In doing so, however, the nucleus precesses around this direction at a characteristic angular frequency (Larmor frequency) which is dependent on the strength of the magnetic field and on the properties of the specific nuclear species (the magnetogyric constant .gamma. of the nucleus). Nuclei which exhibit this phenomena are referred to herein as "spins".
While many different tissue samples and various bodies may be examined using NMR imaging, the invention, for simplicity, is described in the context of an exemplary transaxial volume through a patient's body wherein the volume includes the patient's heart. This volume is herein referred to as a region of interest. In addition, it is assumed that an NMR imaging system includes a three dimensional imaging area having Cartesian coordinate x, y and z axes and that the patient is positioned within the imaging area with the patient's height (i.e. from head to feet) defining an axis along the z axis.
When the region of interest is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the region attempt to align with the polarizing field, but precess about the direction of the field in random order at their characteristic Larmor frequencies. A net magnetic moment M.sub.z is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another.
If, however, the region of interest is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment Mz may be "tipped" into the x-y plane to produce a net transverse magnetic moment M.sub.t which is rotating or spinning in the x-y plane at the Larmor frequency.
The practical value of this phenomenon resides in the signal emitted by the excited spins after the excitation signal B.sub.1 is terminated. The emitted signal is a function of at least one and typically several physical properties of the spin which generates the signal and therefore, by examining the emitted signal, the properties of the spin can be determined. The emitted NMR signals are digitized and processed to generate an NMR data set.
To be useful, an NMR data set requires that the point of origin of each NMR signal sensed be known. To determine the point of origin of an NMR signal, each NMR signal is encoded with special information. An exemplary position encoding technique is commonly referred to as "spin-warp" and is discussed by W. A. Edelstein et al. in "Spin Warp NMR Imaging and Applications to Human Whole-Body Imaging", Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980) which is incorporated herein by reference.
In the spin-warp technique, special encoding is accomplished by employing three magnetic gradient fields (G.sub.x, G.sub.y, and G.sub.z) which have the same direction as polarizing field B.sub.0 and which have gradients along the x, y and z axes, respectively. By controlling the strength of these gradients during each NMR cycle, the spatial distribution of spin excitation can be controlled and the point of origin of the resulting NMR signals can be identified.
A generally useful acquisition technique is known as a slice or two dimensional technique wherein NMR data are acquired for each of several single transaxial slices of a region of interest consecutively, and then the slices are "stacked" together to form a three dimensional data set.
To determine the z-axis origin of a signal, signal generation during slice data acquisition is limited to a specific transaxial slice of the region of interest, at one time, using gradient field G.sub.z. To this end, the Larmor frequency F of a spin can be expressed as: EQU F=(B.sub.0 +B.sub.z).gamma. (1)
where B.sub.z is essentially the strength of gradient G.sub.z within a specific transaxial slice of the region of interest. Because the gradient field strength varies along the z-axis, each z-axis slice has a different Larmor frequency F. When the excitation signal B.sub.0 is provided at a specific excitation frequency, only those spins within the "selected" z-axis slice which are at the excitation frequency are tipped. Therefore, when the excitation signal B.sub.0 is turned off, only spins within the selected z-axis slice generate NMR signals.
A similar technique is used to spatially encode NMR signals along the x axis. To this end, instead of providing a single excitation signal B.sub.0 frequency, excitation signal B.sub.0 is provided at a small range of frequencies. The x axis gradient G.sub.x is small enough that all of the spins along the x axis have Larmor frequencies within the small range of excitation signal frequencies and therefore each of the spins along the x axis generates an NMR signal when the excitation signal is turned off, each x-axis NMR signal having a unique and identifiable frequency. Hence, x-axis position can be determined by identifying the frequency of each NMR signal received during an acquisition. Because x axis position is encoded using signal frequency, this type of encoding is known as frequency encoding.
To encode y axis position within NMR signals the y axis gradient G.sub.y is employed to cause spins along the y axis to have different phases. Consequently, NMR signals resulting from spins along the y axis have different phases which can be used to determine y axis position. Because y axis position is encoded using signal phase, this type of encoding is known as phase encoding.
After data have been acquired for one region of interest slice, the acquisition process is repeated for adjacent regions of interest slices until data have been acquired for every slice within the region of interest. After digitizing and processing, the slice data are combined to provide a three dimensional data point (TDDP) array representing one or more physical properties at regular grid positions within the interior of the region of interest. The TDDP array includes a plurality of sets of three dimensional (x,y,z) coordinates distributed at regular positions in a lattice within the region of interest, at least one value (Vxyz) of the physical property being associated with each respective one of the coordinate positions. Each cubically adjacent set of eight such positions defines a cubic volume, or "voxel", with a physical property value specified for each of the eight voxel vertices.
After a complete TDDP array has been acquired and stored, the array can be used to form an image of the region of interest using one of many well known reconstruction techniques. Typical imaging screens used to display NMR images are only two dimensional. Thus, while shading and the like can give the appearance of a three dimensional image, in reality only two dimensions of pixels can be displayed at any given time. This hardware constraint requires that certain decisions be made as to what aspects of the TDDP array are important for examination purposes.
For example, assume a TDDP array is observed from a specific perspective "viewing angle" wherein array data point columns are perpendicular to, and along the line of sight of, the viewing angle. In examining data points along one of the columns, if a bright data point is behind a dim data point, then, from the perspective view, the bright data point would be "hidden" and valuable information in the image might be lost. This is true of each of the data point columns. This problem is exacerbated because NMR systems generate an appreciable amount of electromagnetic noise which is reflected in a TDDP array, and a perspective view including data point intensities from only the most proximate array within data point columns would be relatively useless as many of the intensities correspond to noise. Consequently, in most cases after array data has been collected and stored, a subset of data is selected for generating an image. For example, one useful visualization technique is known as a maximum intensity projection (MIP). To form a MIP, a specific array viewing angle is selected wherein data point columns are along the viewing angle line of sight. For each column, a processor selects the highest intensity data point in the column and provides that data point in an associated two dimensional array of data points for display on the imaging screen. This MIP technique is valuable in that the MIP image is relatively noise free (i.e. is not dominated by noise) and provides an image which is akin to an x-ray.
Another useful visualization technique is to select a transaxial slice through an NMR data set which is parallel to one of the x, y and z axes so that a cross sectional view of the data, and hence the region of interest, is obtained. This cross section technique allows a physician to observe the detailed spatial relationship between internal structures within the region of interest for diagnosing and prescribing purposes.
One other useful visualization technique is known as oblique reformatting. The industry has generally recognized that in many instances it is desirable to select a cross sectional slice through an NMR data set which is orthogonal to a structural interface and which may form some oblique angle (hence the phrase "oblique imaging") to the orientation of the data acquisition slices. For example, it may be advantageous to observe the length of a vessel which traverses various x, y and z coordinates within the three dimensional data array.
Cline et al. U.S. Pat. No. 4,984,157, "System and Method for Displaying Oblique Planar Cross Sections of a Solid Body Using Tri-Linear Interpolation To Determine Pixel Position Data", issued Jan. 8, 1991 and assigned to the instant assignee (hereinafter "the '157 patent"), is incorporated herein by reference. The '157 patent teaches one method and apparatus for selecting oblique reformatting planes and thereafter converting data point intensities to pixel intensities for display in the oblique image plane.
As an alternative to generating a TDDP array and oblique reformatting thereafter to generate oblique images, oblique image data can be acquired initially via an oblique slice through a patient's body and the acquired data can then be used, without reformatting, to generate a desired oblique image. Methods to acquire oblique image data are well known in the art.
Unfortunately, even conventional oblique imaging techniques have several shortcomings. One shortcoming of oblique imaging is that many vessels are tortuous, so that the vessel is not neatly contained within a single imaging plane. For example, the coronary arteries which are formed on an external surface of the heart are tortuous and multi-planar. In this instance, while a first portion of a vessel may be imageble via selection of a proper oblique imaging plane, other portions of the vessel which lie in different planes cannot be imaged along with the first portion. In addition, where a selected oblique plane passes through one or more heart chambers which include blood pools, the blood obfuscates the resulting image.